The present invention relates to a semiconductor device, and more particularly, it relates to a semiconductor device having a field effect transistor using two-dimensional electron gas.
The “high-electron mobility transistor (HEMT)” is known as one kind of a field effect transistor (FET). The high-electron mobility transistor is a transistor in which, for example, a heterojunction between a channel layer consisting of a high purity semiconductor and an electron supply layer whose electron affinity is smaller than that of the channel layer and in which n-type impurities are doped in high concentration is formed on a semi-insulating semiconductor substrate, and the 2-dimensional electron gas (2DEG) which is built up in the channel layer and has a high mobility of the electron is used as a carrier.
Currently, most general HEMT is Pseudomorphic-HEMT (P-HEMT) in which a gallium arsenide (GaAs) is used as the substrate and an indium gallium arsenide (InGaAs) which does not have a lattice conformity to GaAs is used as the channel layer. InxGa1−xAs which is the material of the channel layer has the advantage that the electron mobility becomes higher, the frequency becomes higher and a noise becomes lower as the indium composition x increases.
However, in P-HEMT, channel layers whose indium compositions x were less than 0.25 have only been used since it was necessary to make the thickness of the channel layer less than a critical thickness (thickness at which a crystalline lattice can maintain its elastic deformation).
On the other hand, in the case of HEMT (InP-HEMT) using the indium phosphorus (InP) as the substrate, an excellent performance of high frequency is obtained since In0.53Ga0.47As has a lattice conformity to InP can be used as the material of the channel layer. However, there are problems that (1) InP substrate is more expensive than GaAs substrate and (2) when the indium composition is 0.53, an energy band gap Eg becomes small and the breakdown voltage to an electric field becomes low.
In recent years, HEMT (Metamorphic-HEMT (MM-HEMT)) in which a semiconductor layer which has a larger lattice constant than GaAs is formed firstly on a GaAs substrate as a buffer layer, and thus, a hetero structure where the InxGa1-xAs layer whose indium composition x is more than 0.3 acts as the channel layer is laminated, has become able to be manufactured by advanced crystal growth technologies, such as a molecular beam epitaxy (MBE) method and a metal organic chemical vapor deposition (MOCVD) method.
MM-HEMT has the advantage of not only being excellent in its high frequency performance since the indium composition of the channel layer can be made larger than that of P-HEMT, but also being less expensive to manufacture than InP-HEMT for the less expensive GaAs substrate.
FIG. 7 is a schematic diagram showing an example of the cross-sectional structure of MM-HEMT which was examined by the Inventor of the present invention in the course of attaining this invention. This structure will be explained as follows along with the manufacturing process.
First, a non-doped InvAl1−VAs buffer layer 902 whose indium composition v is changed gradually from 0 to 0.39, a non-doped In0.40Ga0.6As channel layer 903, a non-doped In0.39Al0.61As spacer layer 904, a Si-doped n-type In0.39Al0.61As electron supply layer 905, a non-doped In0.39Al0.61As Schottky contact layer 906 and a Si-doped n-type In0.40Ga0.60As ohmic contact layer 907 are formed on a semi-insulating GaAs substrate 901 in this order by the MOCVD method.
Next, a source electrode 908 and a drain electrode 909 by a non alloy type ohmic contact in which Titanium (Ti), platinum (Pt) and gold (Au) are laminated in this order are formed by a photolithography and a vapor deposition process.
Next, the n-type InGaAs ohmic contact layer 907 which have only one part exposed by electron beam exposure is etched and removed, and the surface of the non-doped InAlAs Schottky contact layer 906 is exposed. Then, the Ti/Pt/Au gate electrode 910 is formed on it, and the principal part of HEMT is completed.
Although the case that indium composition of the channel layer 903 is 0.4 was explained above in the example, MM-HEMT can be made by the same manufacturing method also in the case of other indium compositions.
Thus, MM-HEMT has an advantage that indium composition of the channel layer is not restrained by the lattice conformity conditions to the substrate. Furthermore, MM-HEMT whose indium composition is in a range between 0.31 and 0.45 has the following advantages:
(1) The band discontinuous quantity ΔEc between the conduction bands of the InAlAs electron supply layer 905 and the InGaAs channel layer 903 is 0.62-0.80 eV, and is larger than 0.34 eV of P-HEMT and 0.52 eV of InP-HEMT. Therefore, high 2DEG concentration is obtained.
(2) Since the band gap Eg of the channel layer 903 is 0.84-1.00 eV and is larger than that of InP-HEMT which is 0.76 eV, a high breakdown voltage is obtained.
However, MM-HEMT whose indium composition is in a range between 0.31 and 0.45 has the following problems:
That is, the band discontinuous quantity ΔEc of the conduction bands at the hetero interface between the n-type InGaAs ohmic contact layer 907 and the non-doped InAlAs Schottky contact layer 906 is 0.62 through 0.80 eV. In contrast, the band discontinuous quantity ΔEc between the GaAs ohmic contact layer of P-HEMT and an AlGaAs electron supply layer is 0.15 eV.
Moreover, the band discontinuous quantity ΔEc between the InGaAs ohmic contact layer of InP-HEMT and an InAlAs Schottky contact layer is 0.52 eV. That is, the band discontinuous quantity of the conduction bands at the hetero interface between the ohmic contact layer 907 and the Schottky contact layer 906 of MM-HEMT is larger than that of P-HEMT and InP-HEMT by 0.1 eV or more.
The band discontinuity of the conduction bands serves as a barrier for the electron current among the source electrode 908, the drain electrode 909 and the InGaAs channel layer 903 by non-alloy ohmic contact. Therefore, the band discontinuity of this conduction band becomes the cause of increasing source resistance and drain resistance remarkably. Consequently, there was a problem that the high frequency performance and low noise performance of the element were degraded.
As explained above, there was a problem that high frequency performance and low noise performance were degraded because the band discontinuous quantity ΔEc between the conduction bands of the n-type InGaAs ohmic contact layer 907 and the non-doped InAlAs Schottky contact layer 906 was large in MM-HEMT expressed in FIG. 7.